Ice movement is one of the most important environmental parameters considered in off-shore arctic platform and island design. Ice movement data, preferably collected over several years, is first required to determine the risk of ice movement in a given area. In limited ice movement areas, where a non-ice movement resistant structure can be used to drill the well, long-term ice movement data is then useful to design safe, cost-effective ice movement contingency systems. Data on ice movement magnitude and direction may be needed to determine the number of ice defense slots around a structure, or the size of an ice-free moat. Rate information is needed to formulate a safe maintenance program in the event the surrounding ice sheet does move toward the platform.
In areas where appreciable ice movement is known to occur, ice movement data are required to determine design ice loads. Ice movement rates are important as the ice failure strength is highly dependent on the ice strain rate. Ice movement magnitude data are needed to assess the risk of ice ride-up on the structure's working surface, or pile-up against the structure. After such pile-ups of rubble pile, freeze and consolidate, the structure's effective diameter against the moving ice sheet is increased, thereby increasing the ice loads caused by subsequent ice movement.
Typically, ice movement in ocean waters of 100 foot depth or less has been measured by length/azimuth systems. These systems detect ice movement electro-mechanically by measuring the distance and azimuth between a reference point on the ice and an anchor on the sea floor. A spring-loaded cable reel coupled to a multi-turn potentiometer provides a measure of the amount of cable stretched between the ice station and the reference anchor. The direction of the ice station relative to the anchor is sensed by a directional vane coupled to the distance-measuring cable below the ice mass. A continuous potentiometer coupled to the directional vane by means of an azimuth tube provides an electrical signal that is a measure of the relevant direction.
The distance-measuring cable is connected to the anchor on the ocean floor by passing it through the ice. The traditional method for allowing movement of the cable and azimuth tube through the ice is to pass the cable and azimuth tube through a larger guide tube filled with gelled material so that the cable is afforded axial movement and the azimuth tube is afforded axial rotation through the guide tube. The gel-filled guide tube is not sealed; the gel itself is relied upon to serve as a seal against entry of water into the guide tube. The guide tube extends through the ice and is typically fixedly attached thereto.
Problems with this arrangement have arisen due to the introduction of water into the guide tube from tidal action. Freezing of the water prevents the turning of the azimuth tube. The gelled material itself presents additional problems in that the gel comprises a diesel oil base which is environmentally objectionable. Additionally, handling of the gel in the field presented safety hazards.
Accordingly, it has remained desirable to develop an apparatus to measure ice movement which is not subject to inaccuracies caused by water freezing, which is environmentally acceptable and which is safe to handle. The solution to these problems is the subject of this invention.